(1) Field of the Invention
The present invention relates to a method and an active device for treating noise on board a vehicle, and to a vehicle having such a device.
The invention thus relates to the field of treating sound nuisance on board an aircraft.
(2) Description of Related Art
The present invention relates more particularly to noise treatment systems. Reducing noise in aircraft is a problem of increasing severity because of the impact of noise on the comfort and the health of the occupants of an aircraft, whether they be passengers or crew.
Impacts on passenger health are also generally estimated in consideration of metrics corresponding to “doses” of noise, where the amplitude of the metric depends both on the noise level and on the duration of exposure to that noise.
The sound level inside aircraft can be treated by incorporating passive treatment seeking to isolate the occupants from external noise sources and to absorb soundwaves reflected in the cabin.
Furthermore, active control methods seek to generate anti-noise for inducing a bubble of silence. Such active control methods have been found to be effective in reducing low frequency harmonic noise or medium frequency or high frequency noise in localized zones. The development of such active techniques in aircraft presenting harmonic sound signatures is particularly suitable for maximizing efficiency on target frequencies while keeping down extra weight.
Nevertheless, at low frequencies, active noise control presents limits that are generally associated with the ability to generate a sound level that is equivalent to the ambient sound level being controlled (in particular in noisy environments).
Furthermore, at high frequencies, active noise control presents limits associated with the calculation capacity of computers and with the very localized effect of the control. At high frequencies, soundwaves present wavelengths that are short. As a result, an active noise control device operating at high frequencies tends to generate a bubble of silence of dimensions that are small, such that the bubble is very localized.
In addition, present-day active noise control techniques also present difficulties in controlling signals having properties that vary rapidly in time.
Active anti-noise techniques thus generally consist in generating anti-noise in the form of a soundwave for attenuating the noise. Such anti-noise is sometimes referred to as “secondary” noise in contrast to the noise that is to be treated, which is sometimes referred to as “primary” noise.
A distinction is generally drawn between “feedforward” techniques that predict or anticipate noise by using a reference primary sensor for sensing noise that is correlated with the source of primary noise that is to be controlled in order to be able to anticipate variations in the primary noise, and “feedback” techniques in which such variations are compensated after they have appeared.
Feedback techniques use algorithms that are known to the person skilled in the art, e.g. the recursive least mean square algorithm or the internal model control algorithm, known by the acronyms RLMS or IMC, whereas feedforward techniques use algorithms that are known to the person skilled in the art, e.g. the filtered reference least mean square algorithm, known by the acronyms LMS or F×LMS.
Furthermore, in this technical field, it is possible to use a device for measuring local noise. The device for measuring local noise seeks to determine the noise that results from combining the secondary and primary noises.
This local noise measurement device may include a local noise sensor that is referred to by the person skilled in the art as an “error” sensor. As a function of the measured local noise, the treatment unit then adapts the coefficients of the filter implemented by the algorithm in use.
Thus, a computer prepares a control signal for a sound actuator while taking into consideration a transfer function that evaluates a natural sound filter associated with the environment of an individual.
It should be recalled that a transfer function serves to establish a mathematical relationship between a noise emitted at a given location and the noise that results at some other location, and more concretely a link between the secondary noise emitted by sound actuators and the determined local noise. Such a transfer function is sometimes referred to by the person skilled in the art as a “secondary” transfer function.
The transfer function thus depends on the path followed by the anti-noise between a sound actuator and an error sensor, for example. One known solution consists in identifying the transfer function in real time by an “inline identification” method.
Although those prior art systems are indeed effective, incorporating them in a rotorcraft encounters problems that are complex, associated in particular with the high level of the noise to be attenuated, the large number of “spectrum” lines, sometimes referred to as tone components, in the spectrum of the noise to be attenuated, which spectrum lines are situated in a frequency band extending up to about 10,000 hertz (Hz), and to the presence of broadband noise in such a spectrum, in particular at frequencies situated in a range going from 10 Hz to 1000 Hz.
According to the document “Selection of active noise control strategy: two test cases” by Jari Kataja et. al, Joint Baltic-Nordic Acoustics Meeting 2004, Jun. 8-10, 2004, psychoacoustic phenomena should be taken into account when selecting the technique to use.
Patents FR 2 769 396 and U.S. Pat. No. 6,224,014 propose reducing spectrum line noise inside a helicopter by controlling a sound actuator as a function of measurements delivered by a sound or vibration sensor.
Those documents concentrate on reducing spectrum line noise coming from mechanical members. That type of control thus treats only a portion of the noise and does not provide effective three-dimensional control for all of the passengers in a cabin.
Proposals are made in patents FR 2 802 328 and U.S. Pat. No. 6,502,043 also to make use of a (reference) sensor that makes measurements correlated with a noise source, and to weight the noise measurement signals in order to give priority to determined zone of the aircraft, e.g. in the proximity of the passenger seats.
The sound actuators used may be loudspeakers or piezoelectric actuators, and the sensors may be microphones or accelerometers. The algorithms used for minimizing noise may be of the feedforward or of the feedback type.
Nevertheless, that solution does not make it possible to track movements of the passengers' ears, and thus to adapt the zone in which control is effective in real time.
Patent EP 1 031 136 describes an active system for attenuating noise inside the cabin of a helicopter having a main gearbox and legs or bars securing the gearbox to the structure of the cabin. The system includes a plurality of actuators linked to each of the legs for applying “opposing-vibrations” to each of the legs, in order to reduce the vibrations that give rise to noise coming from gears of said gearbox, at a frequency that is close to 700 Hz.
That solution has a limited frequency range and it is above all limited in its three-dimensional effectiveness. At such frequencies, it is possible to obtain optimized overall control for all passenger positions solely by canceling vibration in the gearbox legs. Transfers between the legs and passenger positions differ from one passenger position to another. The only way to obtain the same effectiveness on the noise derived from vibration transfer regardless of passenger position would be to cancel vibration completely in each leg.
Nevertheless, there remains noise resulting from transfer through the air, which noise is not controlled by the actuators. Unfortunately, this noise due to transfer by air is not negligible in helicopters. Furthermore, the transfer of this noise likewise differs from one position to another.
Furthermore, the solutions proposed in the above-mentioned patents deal solely with spectrum line noise from mechanical systems and therefore they do not enable “broadband” noise to be reduced, which noise might be of aerodynamic origin.
Patent U.S. Pat. No. 5,845,236 proposes using an active attenuator device in addition to vibratory resonators.
Patent U.S. Pat. No. 5,754,662 proposes treating low frequency noise and noise at frequencies higher than the low frequencies separately, and controlling two actuators separately, one adapted to the low frequencies and the other to the higher frequencies. Proposals are made in particular to use a sub-woofer for low frequencies.
Patent EP 0 917 706 describes a noise attenuation system adapted to a twin-engined airplane.
That patent describes treating structural vibration of an aircraft, and consequently treating the sound radiation source of the structure via vibratory actuators placed on the structure.
Patent FR 2 899 011 describes a channel separation control device using a member for psychoacoustically weighting the received and/or emitted signals.
One channel is dedicated to controlling very low frequencies via a sound actuator having an optimized vent or electromechanical vibrator. Another channel is dedicated to controlling medium and high frequencies via microphone-and-loudspeaker pairs at passenger positions.
The microphones are fastened to the seats, and the loudspeakers are also stationary.
Furthermore, nothing makes it possible to take into consideration the influence of movement of a passenger's head on the noise measurement and on the transmission of the anti-noise. The three-dimensional zone in which noise control is effective at high frequencies can then be limited and the bubble of silence that is created does not follow the passenger.
Patent U.S. Pat. No. 4,977,600 descries an active seat using a set of microphones in a headrest as error microphones in order to generate a control zone of defined shape. That patent also describes the possibility of using a plurality of sound actuators for separate frequency bands.
Patent U.S. Pat. No. 6,343,127 describes an active sound control system that positions sound actuators in an enclosure.
Document WO 03/073415 describes another system in which variable signals are weighted over time in order to avoid saturating the actuators.
Document WO 00/14722 provides an integrated active noise control system, with a passenger adapting the position of an assembly comprising a sound actuator and a sensor in order to improve performance.
Document FR 2 899 011 seeks to solve such problems with the help of a device comprising:                at least one local noise sensor such as a microphone situated in a cabin;        at least one loudspeaker;        at least one treatment unit arranged to receive noise measurement signals delivered by the microphone(s), and to deliver control signals to the loudspeaker(s) in order to attenuate noise in the cabin; and        at least one sound resonator acoustically coupled to a loudspeaker and to the cabin.        
The treatment unit then issues a control signal for generating a secondary noise suitable for opposing a primary noise, and it sends this control signal to the loudspeaker and to the resonator.
The secondary noise is prepared with the help of a feedforward or feedback type algorithm by taking into consideration a local noise signal coming from the local noise sensor, and where appropriate a reference primary noise signal coming from a reference primary noise sensor.
Thus, the treatment unit instructs the sound actuator to generate secondary noise that is optimized to be in phase opposition with the primary noise.
The treatment unit may seek to minimize the local noise as measured in decibels (dB) or to minimize a predetermined function that takes passenger comfort into account by using a psychoacoustic approach.
That device is particularly effective in reducing harmonic noise at low and medium frequencies by creating a bubble of silence over a large area.
It is possible to envisage adapting the device to enable it to reduce high frequency noise. Unfortunately, the loudspeaker would then create a bubble of silence of small size, while generating additional noise that is particularly troublesome outside the bubble of silence.
Under such circumstances, the device does not appear to be suitable for high frequency noise. It should be observed that such high frequency noise is particularly troublesome insofar as it lies in a frequency range corresponding to maximum sensitivity of a human ear.
In addition, the movement of an individual's head has an impact on the response of the actuators by modifying the path over which the anti-noise is transmitted, referred to as the secondary path.
Furthermore, it should be observed that Document U.S. 2004/0170286 provides for a system serving to adapt a sound device of a vehicle as a function of the presence or absence of people in the vehicle, such presence being detected by a shape recognition system.
Document U.S. Pat. No. 6,553,296 describes a device capable of determining the macroscopic presence and the relative position of an occupant in a vehicle.
Those two documents do not appear to be capable of solving the new problem relating to reducing or eliminating the trouble created by high frequency noise as a function of the position of an individual's ears relative to sound actuators.
Document EP 2 119 627 proposes sliding a sound actuator support in a cavity placed in a seat back, the sliding relating to the inclination a passenger requests of the seat proper of that seat.
The seat also has a shell surrounding the passenger. Such a shell can be troublesome in a cabin of small dimensions, such as a rotorcraft cabin, for example.
Document FR 2 808 371 describes an active control device incorporated in the headrests of seats and making it possible remotely to measure sound in the proximity of an ear.
Also known are Documents WO 96/31872 and DE 10 2011 013343.